The present application is a U.S. non-provisional application based upon and claiming priority from Japanese Application No. HEI 10-348852 is hereby incorporated by reference.
The present invention concerns a polycarbonate resin for optical use, and more specifically concerns a polycarbonate resin for optical use that has excellent color tone stability and forming properties, and is particularly suited for the substrates of optical disks.
Polycarbonate has excellent mechanical properties such as impact resistance, and it also has excellent heat resistance and transparency; therefore, it is widely used for the parts of many kinds of machines and equipment, optical disks, automobile parts, etc. Research has been conducted extensively in great anticipation of its application to optical uses, particularly optical disks for memory storage, optical fibers, lenses, etc.
Well-known methods for manufacturing polycarbonate include a process in which a bisphenol such as bisphenol A is reacted directly with phosgene (interface method) and a process in which a bisphenol such as bisphenol A undergoes a melt polycondensation reaction with a carbonic acid diester such as diphenyl carbonate (ester substitution reaction).
Among these methods, the interface method that uses phosgene requires large quantities of solvent such as methylene chloride, and because the removal of the chlorine is extremely difficult, the polycarbonate produced is not always satisfactory for optical use.
On the other hand, one advantage of the melt polycondensation reaction method is that polycarbonate can be manufactured relatively inexpensively compared with the interface method. Moreover, because it does not utilize toxic substances such as phosphene or large quantities of a solvent such as methyl chloride, it shows great promise as a manufacturing method for polycarbonate for optical use.
However, a polycarbonate resin composition that can be used for optical applications must have excellent color tone stability and not be discolored by heating during the forming process.
The addition of various types of phosphorous acid ester compounds to the polycarbonate has been proposed in JP Kokai 98-60247 and JP Kokai 92-81457 as attempts to improve the color tone stability of polycarbonate during the forming process, but both these procedures concern polycarbonate that is obtained by the interface method that utilizes phosgene, and when these procedures were applied without modification to the melt polycondensation method, sufficient improvement in the color tone stability of the polycarbonate resin could not be obtained.
Therefore, the applicants have discovered that a polycarbonate with improved color tone stability can be obtained by the addition of an acidic compound to the reaction product obtained in a melt polycondensation process, and have proposed this solution in JP Kokai 92-175368, JP Kokai 92-328156, and JP Kokai 93-262969.
However, although these kinds of polycarbonate resins have color tone stability at relatively low temperatures, they are not always satisfactory for applications that require excellent color tone stability and working properties at higher temperatures such as the high density DVD and DVD-R disks that have been developed in recent years, and therefore an improvement was needed.
The inventors considered this problem carefully, and after diligent research discovered that a polycarbonate resin composition with excellent color tone stability at high temperatures can be obtained by the further addition of a specified amount of acidic compound when various types of lubricants are blended into the polycarbonate, thus completing the present invention.